In oil and gas development engineering, shale may serve either as a reservoir subjected to repeated hydraulic fracturing or as a caprock subjected to long-term gas injection and production in underground gas storage. In both cases, it is exposed to the coupled effects of cyclic loading and fluid invasion. Therefore, clarifying the deformation behavior, damage accumulation, and instability mechanisms of shale under such coupled conditions is of great significance for understanding its mechanical response and evaluating its engineering stability. In this study, Fuling shale was selected as the research object. Uniaxial monotonic loading tests and graded cyclic loading tests were conducted under three conditions, namely dry, oil-saturated, and water-saturated states. In addition, cyclic loading tests within a high-stress range were carried out under the water-saturated condition. Through these tests, the evolution laws of shale strength, deformation, residual strain, and energy dissipation under different fluid conditions were systematically analyzed, and the effects of fluid state on the cyclic mechanical behavior of shale were further compared. The results show that dry shale exhibits the highest overall strength, whereas water saturation significantly weakens both the strength and stiffness of shale and markedly increases the proportion of energy dissipation during deformation and failure. Oil saturation has only a limited influence on the elastic modulus, but it enhances the residual deformation and energy dissipation during cyclic loading. Under all three fluid conditions, cyclic instability occurs when the stress level is still lower than the peak stress under monotonic loading, indicating that shale may become unstable before reaching its monotonic peak strength under repeated loading. Meanwhile, the corresponding peak total strain at instability is close to that at monotonic failure. Before the onset of instability, the growth rate of residual strain shows a pronounced increasing trend, reflecting the accelerated accumulation of irreversible deformation. Under all three conditions, the evolution of cyclic residual strain exhibits clear stage characteristics: the deformation in the first cycle is relatively large, then decreases and tends to stabilize, and finally increases rapidly again as failure approaches, showing an obvious stage-dependent pattern. These findings indicate that, compared with traditional evaluation methods based only on peak stress or uniaxial compressive strength, the criteria based on total strain, the growth rate of residual strain, and the energy dissipation ratio are more suitable for evaluating wellbore stability during shale oil and gas development and during long-term gas injection and production in underground gas storage. Therefore, these parameters can provide more appropriate indicators for stability assessment under coupled cyclic loading and fluid invasion conditions.

The problem of volume loss and cavern damage of salt cavern gas storages has been studied over many years. Using the Jintan salt cavern gas storage X as an example, a three-dimensional geometrical model was established according to the sonar cavity test results, and a nested double meshing was applied. Numerical simulation of gas storage operations over 10 years has been carried out. By studying the volume deformation and damage of salt cavities in different operating years under periodic injection and production conditions, the weak area of cavity stability during gas storage operation is predicted. The results show that compared with the upper part of the cavity, the displacement deformation of the smooth position of the cavern waist is smaller; With the continuous increase of working life, the displacement and deformation of the cavity wall at the same position gradually increase, and the rate of deformation increase gradually decreases; After 10 years of operation, the volume loss rate of gas storage reaches about 7%, and the volume loss of gas storage tends to slow down in the following years; The protruding position of the cavity wall from the middle of the salt cavity to the top plate is the weak area of the stability of the gas storage cavity, which should be avoided in the cavity design of the salt cavern gas storage.

Due to the density contrast between the hydrate and methane gas, the pore pressure is accumulated in the sediment during the decomposition process of methane hydrate. This accumulation of pore pressure decreases the magnitude of effective stress, further triggering potential geological disasters such as landslide. This paper establishes a theoretical framework to investigate the evolution of fluid pressure in the hydrate-bearing sediments during the decomposition process. This model consists of two parts: an unsaturated thermo-poromechanical constitutive law as well as a phase equilibrium equation. Compared with the existing studies, the present work incorporates the effect of pore volume change into the pressure built-up model. In addition, the capillary effect is considered, which plays a nontrivial role in fine-grained sediments. Based on this model, the evolution of fluid pressure is investigated in undrained conditions. It is shown that four mechanisms mainly contribute to the pressure built-up: the density contrast between decomposing hydrate and producing fluid, the variation of pore volume, the compaction of hydrate due to variation of capillary pressure, and the thermal deformation of pore constituents induced by temperature change. Among these mechanisms, the density contrast dominates the pore pressure accumulation. Under the combined effect of these contributions, the evolution of fluid pressure exhibits a strong nonlinearity during the decomposition process and can reach up to dozens of mega Pascal. Nevertheless, this high-level pressure built-up results in a significant tensile strain, yielding potential fracturing of the sediment.